Ceramic multi-layer element and a method for the production thereof

ABSTRACT

A method of manufacturing a ceramic component having multiple layers includes producing ceramic green foil comprised of PTC ceramic material, applying electrode paste containing tungsten onto areas of the ceramic green foil designated to be electrodes, alternately stacking a number of ceramic green foils with electrode paste to produce a foil stack, compressing the foil stack, and sintering the foil stack to produce a component body.

FIELD OF THE INVENTION

The invention relates to a ceramic multilayer component according to theintroductory clause of claim 1, as well as a method of manufacturing thecomponent.

Such a component is known from EP 0734031A2. This type of componentcomprises a monolithic ceramic component body made of a perovskiteceramic material that has a multilayer structure consisting ofalternating ceramic and electrode layers. The interior electrodes, whichare based on nickel or nickel alloys, are alternately connected tocollector electrodes attached to the exterior of the component body. Thecomponent is structured as a varistor.

A ceramic multilayer component that can be used as a capacitor is knownfrom U.S. Pat. No. 3,679,950. This component also features alternatingceramic and electrode layers, wherein the electrode layers arealternately bonded with two collector electrodes laterally connected tothe component body. During the production of the ceramic component, theelectrode layers are initially pre-manufactured as intermediate porousceramic layers and only afterwards impregnated with conductive material,such as silver in a silver nitrate melt or in a melt of a BiPbSnCdalloy.

With the exception of the complex process mentioned above, the onlyceramic/electrode combinations suitable for the production of ceramicmultilayer components are those that survive sintering into denseceramic component bodies at temperatures normally ranging from1200–1500° C.

None of the temperature-stable electrodes made of precious metals thatare normally used are suitable for use with ceramic posistors, i.e.,components with a positive temperature coefficient of resistance, orso-called PTC elements. These components cannot develop an ohmic bondbetween the ceramic material and the metallic electrodes. Consequently,PTC elements with (interior) electrodes made of precious metal exhibitimpermissibly high resistance. However, the non-precious metals suitablefor use as electrode material generally do not survive the sinteringprocess, which is necessary for the development of multilayercomponents.

DE 197 19 174 A1 discloses a ceramic posistor with a multilayerstructure that features electrode layers comprising aluminum. Theseelectrode layers develop an ohmic bond to the ceramic and can besintered without damage at temperatures of up to 1200°. A disadvantageof this multilayer posistor component, however, is that the aluminumfrom the electrode layers partially diffuses into the ceramic material,thereby impairing the component properties in the medium or long-term oreven rendering the component unusable.

DE 196 22 690 A1 discloses a ceramic multilayer component comprising astack, bonded together to form a monolithic component body, of severalceramic layers provided with electrodes on both sides, wherein theelectrode layers are alternately bonded with collector electrodeslaterally connected to the component, and wherein the material of theinterior electrode comprises tungsten.

SUMMARY

The object of the present invention is to specify a ceramic multilayercomponent with ceramic layers comprising PTC ceramic, wherein thecomponent exhibits interior electrodes that are stable with respect tosintering and possesses component characteristics of long-termstability.

According to the invention, this object is achieved by a ceramicmultilayer component of the type mentioned initially, in which thematerial, at least of the interior electrodes, comprises tungsten, andin which the ceramic layers comprise a PTC ceramic material.

Advantageous embodiments of the invention as well as a method formanufacturing the component follow from the additional claims.

It has been shown that electrodes made of tungsten or containingtungsten survive the sintering process necessary for the ceramiccomponent without damage and, during this process, develop a favorableohmic bond to the PTC ceramic material. Consequently, components withlow resistance can be obtained with the invention. No diffusionprocesses of tungsten in the ceramic material that could impair theceramic component characteristics are observed during sintering. Thisalso applies to ceramic posistors, which also develop a favorable ohmicbond to the electrodes comprising tungsten without posistorcharacteristics being lost. At the same time, tungsten exhibits goodelectric conductivity, which is comparable with that of precious metalsand is about three times as high for tungsten as for silver, so thatelectrode layers with adequate electric conductivity can already beachieved with thinner tungsten layers, as has thus far been possiblewith known non-precious electrode layers. Moreover, tungsten is acost-effective electrode material which, for example, is significantlymore cost-effective than precious metals such as palladium or platinum,so that the ceramic multilayer components of the invention can bemanufactured more cost-effectively than those with electrodes containingmetal. According to the invention, however, it is not the electricconductivity of tungsten, but rather removal of a blocking layer ofposistor material, which is achieved only through the presence of asuitable amount of tungsten, that establishes a favorable ohmic bond.

A component of the invention formed as a PTC element, and thereforemanufactured with a posistor ceramic material, provides additionalbenefits that have thus far been unachievable. Since no stable ceramicmultilayer posistors have been known in the art until now, it is nowpossible to produce posistors with higher-rated currents and smallercomponent resistances in a smaller design than was possible in known(single-layer) posistor components. This is possible because, in thecase of multilayer components, distances between electrodes and/or thethicknesses of ceramic layers can be smaller than in conventionalposistor components without interior electrodes. As the thickness of theindividual ceramic layer is reduced, its electric resistance is alsoreduced perpendicular to a main surface, i.e., in a direction of thelayer thickness, without requiring a reduction in the specificresistance of the ceramic material. A further reduction in theresistance of the entire multilayer component results from theconnection in parallel of individual PTC elements which, in thecomponent of the invention, are stacked on top of one another to producethe multilayer component. This also ensures a high current carryingcapacity of the component.

In a ceramic multilayer component, the characteristics of the totalcomponent can generally be specifically influenced or varied by varyingthe parameters of layer thickness and surface area of the individualelement and the number of stacked individual layers in the multilayercomponent. Therefore, the characteristics of a multilayer component withgiven external dimensions can nevertheless be varied within furtherlimits without the need to change the composition of the ceramicmaterial. In the case of single-layer ceramic components, the componentcharacteristics can often be varied only through variation of thecomponent dimension or variation of the materials used to make thecomponent.

Thus, a ceramic multilayer component is especially suitable for use inSMD assembly technology, which requires a compact, machine-processableor machine-capable structural shape. This shape can be varied in anymanner in the multilayer component, as the component characteristics canbe adjusted independently thereof.

In the following, the invention, and particularly the method formanufacturing the component, are described in greater detail on thebasis of exemplary embodiments and corresponding figures. The figuresare provided for the sole purpose of illustrating the invention, and areonly schematic and not dimensionally accurate.

DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an exploded view of a ceramic green foil imprinted withan electrode layer.

FIG. 2 depicts a multilayer component according to the invention inschematic cross-section.

FIG. 3 depicts a ceramic green foil divisible into multiple components,with active and passive areas, in top view.

FIG. 4 depicts a stack of layers of ceramic green foil in cross-section.

DETAILED DESCRIPTION

To produce ceramic foil, a ceramic starting material is finely groundand homogeneously mixed with a binder material. The foil is thenproduced in its desired thickness by means of foil drawing or foilpouring.

FIG. 1 depicts such a green foil 1 in an exploded view. An electrodepaste is now applied to a surface of the green foil 1 in the areadesignated for the electrode.

A number of thick-layer processes, preferably printing, such as by meansof silk screen printing, are suitable for this purpose. An area of thesurface not covered by electrode paste, and designated here as a passivearea 3, remains in at least the region of one edge of the green foil 1,as depicted in FIG. 1, for example, or only in the region of one cornerof the green foil. It is also possible not to apply the electrode as aflat layer, but rather as a structured and possibly intermittentpattern.

The electrode paste 2 consists of metallic particles comprising metallictungsten or a tungsten compound, so as to provide the desiredconductivity, and, if applicable, sinterable ceramic particles to adjustthe shrinkage properties of the electrode paste to those of the ceramicmaterial, and an organic binder, which can be burned off, so as toguarantee moldability of the ceramic material and/or cohesion of thegreen objects. Particles of pure tungsten or particles of tungstenalloy, tungsten compound or mixed particles of tungsten and other metalscan be used in this process. In the case of ceramic multilayercomponents that are only subjected to minor mechanical stress, it isalso possible to completely eliminate the ceramic components from theelectrode paste. The tungsten component can vary within a wide range,wherein it may be necessary to adjust the sintering conditions to thecomposition of the electrode paste. The reduction in the blocking layerin posistor material is routinely achieved with tungsten particles of 3or more percent in weight (relative to the metallic particles).

Subsequently, the desired number of imprinted green foils 9 are stackedon top of one another to form a stack of foils, such that (green)ceramic layers 1 and electrode layers 2 are arranged alternately on topof one another.

During the subsequent bonding process, the electrode layers are alsoalternately bonded to collector electrodes on different sides of thecomponent, so as to connect the individual electrodes in parallel. Tothis end, it is advantageous to stack the first and second green foils 9in different directions on the imprinted electrode layers 2 in such away that their passive areas 3 alternately point in differentdirections. A uniform electrode geometry is preferably selected for thispurpose, wherein the first and second green foil 9 differ from oneanother in that they are rotated by 180° relative to one another withinthe foil stack. It is also possible, however, to select a highlysymmetrical outline for the component, so that rotation by angles otherthan 180° is possible to produce alternate bonding, e.g., by providing asquare outline to enable 90° rotation. However, it is also possible toshift the electrode pattern on every second green foil 9 by a specificamount relative to that of the first green foil, so that each passivearea 3 in the respective adjacent green foil is disposed over an areaimprinted with electrode paste.

Subsequently, the foil stack, which is still elastic with respect toshape due to the binder, is transformed into the desired external shapeby pressing and, if necessary, cutting it to size. Then the ceramicmaterial is sintered, which can encompass a multi-stage process in anatmosphere that, at least initially, contains little oxygen. Finalsintering, in which the ceramic material is sintered together untilcomplete densification and/or desired densification is achieved,generally falls between 1100 and 1500° C. If an atmosphere containingoxygen (with, for example, a partial oxygen pressure of at least 1hectopascal) is selected for this high-temperature sintering step, amaximum sintering temperature of 1200° C. is maintained. At temperaturesabove this level, there is a risk that the tungsten contained in theelectrodes may oxidize, thereby reducing electric conductivity. Ifsintering is performed under inert gas (e.g., with an oxygen partialpressure of no more than 1 pascal), which is also possible, it is notnecessary to maintain this upper temperature limit, so that sinteringcan also be performed at a temperature of 1300° C., which normally usedwith barium titanate, for example. However, a reduction in the requiredsintering temperature can also be achieved by selecting suitableadditives to the ceramic material.

Sintering transforms the individual green foil layers into a monolithicceramic component body 8, which exhibits a solid bond among theindividual ceramic layers 4. This solid bond also exists at theceramic/electrode/ceramic points of connection. FIG. 2 depicts afinished multilayer component 8 of the invention in schematiccross-section. Alternating ceramic layers 4 and electrode layers 5 arestacked on top of one another in the component body. At two oppositepoints in the component body, collector electrodes 6, 6′ are generated,each of which is in electric contact with every other electrode layer 5.To achieve this, a metal coating, generally consisting of silver on theceramic material, can be produced, generally through currentlessprecipitation. This coating can subsequently be reinforced throughelectroplating, e.g., by applying an Ag/Ni/Sn coating sequence. Thisimproves the material's capacity to be soldered onto printed circuitboards. However, other methods of metal coating and/or generation ofcollector electrodes 6, 6′ are also suitable.

The component 8 depicted in FIG. 2 features ceramic layers as finallayers on both main surfaces. To this end, for example, a non-imprintedgreen foil 1 can be incorporated into the foil stack as the top layerprior to sintering, so that the stack does not end with an electrodelayer 2. For ceramic components that are subjected to particularly highlevels of mechanical stress, it is also possible to design the uppermostand lowermost ceramic layers in the stack to be thicker than theremaining ceramic layers 4 in the stack. To this end, severalnon-imprinted green foils 1 without electrode layers can be incorporatedas the uppermost and lowermost layers during stacking of the foil stackand then compressed and sintered together with the remaining green foilstack.

FIG. 3 depicts a green foil imprinted with an electrode pattern 2, whichallows for a division into several components with progressively smallersurface areas. The passive areas 3 that are not imprinted with electrodepaste are arranged in such a way that, through alternate stacking offirst and second green foils, the alternating displacement of theelectrodes suitable for bonding can be adjusted within the stack. Thiscan be achieved if the first and second green foils are rotated relativeto one another by 180°, for example, or if first and second green foilsgenerally feature electrode patterns that are shifted relative to oneanother. The intersection lines 7 along which the green foil or thestack of layers made with the green foil can be separated intoindividual components are indicated by dashed lines. Electrode patternsare also possible in which the intersection lines used for separationcan be designed in such a way that none of the electrode layers must becut apart. In this case, however, every other electrode layer can bebonded beginning at the edge of the stack. If necessary, the stacks areadditionally polished after separation and sintering and prior toapplication of the collector electrodes 6, 6′, so as to expose theelectrode layers to be bonded.

FIG. 4 depicts a stack of layers produced in this manner, in schematiccross-section, which includes ceramic layers 4 and electrodes 5. It isevident that the separation of the stack of layers along theintersection lines 7 results in components, each of which exhibits thedesired displacement of the electrodes 5. The division of such a foilstack comprising several component outlines into individual foil stackshaving the desired component surface area is preferably accomplishedprior to compression of the foil stack by means, for example, of cuttingor punching. The foil stacks are subsequently sintered. However, it isalso possible to initially sinter the foil stacks comprising severaloutlines of components, and then to separate the individual componentsby sawing apart the completely sintered ceramic material. Again,collector electrodes 6 are applied afterwards.

A multilayer component of the invention, which can be used as a posistor(PTC element), comprises a barium titanate ceramic material with thegeneral composition (Ba,Ca,Sr,Pb)TiO₃, which is doped with donatorsand/or acceptors, such as manganese and yttrium.

The component can, for example, comprise 5 to 20 ceramic layers,including the corresponding electrode layers, but at least two interiorelectrode layers. The ceramic layers are normally 30 to 200 μm thick.However, they can also exhibit larger or smaller layer thicknesses.

Although an external dimension of a posistor element in the multilayermethod of construction of the invention can vary, it normally fallswithin the range of a few millimeters commonly used with components thatcan be processed with SMD. A suitable size, for example, is structuralshape 2220, which is commonly used with capacitors. However, theposistor element can also be even smaller.

The process for manufacturing ceramic multilayer components, which, withthe exception of selection of the electrode material, is known in theart, could be described only in exemplary fashion on the basis of theexemplary embodiment. Consequently, the invention is not limited to theexemplary embodiments and can be modified further as desired by varyingmost of the parameters.

The invention is especially advantageous for the posistor componentsmentioned earlier, which, as a result of the invention, can be producedfor the first time as stable multilayer components with small structuralshapes and low resistance levels. However, it is also possible toproduce other ceramic multilayer components with the invention, such ascapacitors, high-temperature conductors, and varistors.

1. A ceramic component having multiple layers, the ceramic componentcomprising: a stack of ceramic layers having internal electrodes; andcollector electrodes connected to the stack; wherein the ceramic layerscomprise PTC (Positive Temperature Coefficient) ceramic material and theinternal electrodes comprise tungsten.
 2. The ceramic component of claim1, wherein the internal electrodes comprise at least two electrodelayers in the stack.
 3. The ceramic component of claim 1, furthercomprising: a top ceramic layer at a top of the stack; and a bottomceramic layer at a bottom of the stack; wherein the top ceramic layerand the bottom ceramic layer do not have internal electrodes.
 4. Theceramic component of claim 3, wherein at least one of the top ceramiclayer and the bottom ceramic layer has a thickness that is greater thanthicknesses of individual ceramic layers in the stack between the topceramic layer and the bottom ceramic layer.
 5. The ceramic component ofclaim 3, wherein each of the top ceramic layer and the bottom ceramiclayer has a thickness that is greater than thicknesses of individualceramic layers in the stack between the top ceramic layer and the bottomceramic layer.
 6. The ceramic component of claim 1, wherein the stackhas a first side and a second side, a first collector electrode isconnected to the first side, and a second collector electrode isconnected to the second side.
 7. The ceramic component of claim 6,wherein, in the stack, an internal electrode for a first ceramic layerconnects to the first collector electrode but not to the secondcollector electrode, and an internal electrode for a second ceramiclayer connects to the second collector electrode but not to the firstconnector electrode, the first ceramic layer and the second ceramiclayer being adjacent in the stack.
 8. The ceramic component of claim 1,wherein internal electrodes of adjacent ceramic layers in the stackconnect to different collector electrodes.
 9. The ceramic component ofclaim 1, wherein the collector electrodes comprise silver andelectroplating.
 10. The ceramic component of claim 1, wherein athickness of a ceramic layer is between 30 μm and 200 μm.
 11. Theceramic component of claim 1, wherein there are between five and twentyceramic layers in the stack.
 12. The ceramic component of claim 1,wherein the ceramic layers comprise one or more of Ba, Ca, Sr and Pb incombination with TiO₃, and the ceramic layers are doped with donatorsand/or acceptors, the donators and/or acceptors comprising at least oneof manganese and yttrium.
 13. The ceramic component of claim 1, whereinthe internal electrodes are substantially parallel.
 14. An SMD (SurfaceMounted Device)-capable PTC (Positive Temperature Coefficient) resistorelement, comprising: a stack of ceramic layers having internalelectrodes; and collector electrodes connected to the stack; wherein theceramic layers comprise PTC ceramic material and the internal electrodescomprise tungsten.